1. Field of the Invention
The invention relates to mixtures comprising polar groups bonded via methylene groups, the mixtures being crosslinkable to give vulcanizates having high relative permittivities at high electric breakdown strengths.
2. Description of the Related Art
There is a need for dielectric elastomers for application in actuators, generators or sensors in robotics, orthopedics and other fields, for electrically controllable “artificial muscles,” transducers which produce electricity from kinetic energy, and also sensors. Intelligent constructions, such as minimal energy actuators or stack actuators with very small layer thicknesses are required to obtain the best possible performance characteristics. There is a great interest in electroactive polymer actuators which are able to convert electrical energy into linear mechanical motion. Although actuators which have been able to perform deflections of more than 100 percent have already been developed, reliable and repeatable relative elongations of 10 to 30 percent, however, are the current state of art. Even these relative elongations require an operating voltage of several thousand volts.
On account of these high operating voltages, which are impractical and unsafe, scientists are researching materials which can also be operated at lower voltages. The activity of an actuator can be improved by enhancing its ability to store the electrical energy density. This corresponds to the increase in the permittivity εr of the active material. Many approaches have led to a deterioration in the mechanical properties and reduced breakdown field strengths, above which the material suffers a catastrophic electrical breakdown.
Dielectric elastomers are suitable as base materials for artificial muscles and actorics applications. Primarily polyurethane (PU) and polydimethylsiloxane (PDMS) elastomers are discussed as possible material classes. Due to the large number of basic structures suitable for the PU and PDMS synthesis, the mechanical properties of the specified polymers can be readily adapted to the requirements of artificial muscles and actoric elements. The permittivity of the specified polymer structures, however, is usually limited to values from about 3 to 10. This gives rise to switching voltages of generally >1 kV for artificial muscles and actoric elements produced from these materials. These high switching voltages place narrow limits on the applicability of this technology. For this reason, various technical proposals and solutions have been made which have lead to a significant increase in the permittivity. Thus, by blending the specified polymers with nanoparticles of high-permittivity inorganic materials, such as barium titanate, lead zirconate, titanium dioxide etc., it is possible to significantly increase the permittivity of the elastomers such that lower switching voltages of several hundred volts can be achieved. However, these solutions have significant disadvantages which lie in a considerable impairment of the processing properties, a change to the mechanical properties of the elastomers (stiffening) and problems of homogeneous distribution of the nanoparticles in the elastomer matrix. Nanoparticles can aggregate, agglomerate and lead to problems in the formation of homogeneous elastomer films. Furthermore, nanoparticles of the specified inorganic materials are generally highly reactive as a result of their high internal surface area and may thus lead to destructuring or damage of the elastomer matrix. Furthermore, the interactions at the nanoparticle/elastomer matrix interface constitute a frequent problem that is not easy to solve when using nanoparticles for increasing the permittivity of dielectric elastomers. Actuators which consist of soft dielectric elastomers become deformed on account of the Maxwell pressure σ (Maxwell), which is induced by the electric field and interacts with the mechanical properties of the material.
      s    z    =                    σ        Maxwell            E        =                                        ɛ            0                    ⁢                      ɛ            r                    ⁢                      E            b            2                          E            =                                    ɛ            0                    ⁢                                                    ɛ                r                            ⁡                              (                                                      U                    b                                    d                                )                                      2                          E            
Herein, sz is the maximum deformation in the z direction, σ(Maxwell) is the electrostatic pressure from the electrodes, E is the modulus of elasticity of the material, ε0 is the permittivity of the vacuum, εr is the relative permittivity of the material, Eb is the breakdown field strength, Ub is the breakdown voltage and d is the thickness of the active material. It can be seen from the above equation that to achieve large elongations and low voltages, it is necessary to increase the relative permittivity and reduce the modulus of elasticity. The thickness of the electrical film can likewise be reduced, but is limited by the technical possibilities. The smallest layer thickness currently that has been realized for actoric purposes is approximately 5 μm.
DE 10 2010 046 343, the disclosure of which is incorporated herein in its entirety, describes that the attempts hitherto to increase the relative permittivity in elastomers by adding additives can only be realized with difficulty and are burdened with major disadvantages.
Dielectric elastomer actuators (DEA) are electroactive polymers which can become considerably deformed by applying an activation voltage. For this reason, they are often also referred to as “artificial muscles”. These components (described for example in U.S. Pat. No. 6,545,384 B1, the disclosure of which is incorporated herein in its entirety), consist, in the simplest case, of an elastic dielectric with the thickness d which is located between two expandable electrodes. This technology has many practical advantages over conventional actuators:                high specific electromechanical energy,        deformation-based movement and consequently continuous, judder-free deflection,        noiseless,        high efficiency as a result of direct coupling to the voltage signal, and        soft materials and consequently low sensitivity to impacts.        
These dielectric elastomer transducers can likewise be used in sensorics and for so-called “energy harvesting” on account of their function principle. Both fields of application have great potential. Particularly in sensorics, there are many different applications which are ripe for the market.
The current disadvantages of these actuator systems are the high operating voltages of several thousand volts. For this reason, materials with high permittivity εr, high breakdown field strength Eb and low modulus of elasticity E are sought in order to increase the maximum expansion sz and to lower the operating voltage U.
Starting from this, it is therefore an object of the present invention to provide a polymer material that is suitable as a dielectric and has a high relative permittivity and high breakdown strength (breakdown field strength) which is easy to access in a cost-effective manner. Moreover, the polymer material should have high media resistance to be able to be used for the most diverse possible applications.
Polydiorganosiloxanes with polar functional groups have already been recognized as potentially suitable candidates. For example, DE 10 2010 046 343, the disclosure of which is incorporated herein in its entirety, describes siloxane additives, which are very complex to produce, for increasing the relative permittivity in (addition-crosslinking) silicone mixtures. JP 49080599, the disclosure of which is incorporated herein in its entirety, indicates that chloromethylmethylsiloxane units in linear siloxanes lead to a significant increase in the relative permittivity up to 6.2 (50 Hz) for a simultaneously high breakdown strength (41 kV at 2.5 mm).